1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more specifically to a lithographic projection apparatus including a controllable reflector.
2. Background of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (XUV or EUV) or soft x-ray and possible sources include laser-produced plasma sources, discharge plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Lithographic projection apparatus using EUV radiation are intended to image mask patterns with a critical dimension of 90 nm or less. This imposes extremely severe accuracy criteria on the illumination and especially the projection optics. For the projection system, the required accuracy is defined by the wavefront aberration (WFA) which is twice the magnitude of the surface figure error. For a four-mirror system it has been calculated (Gwyn, C. W. et al, Extreme Ultraviolet lithography, J. Vac. Sci. Technol. B 16, (November/December) 1998, pp 3142) that a WFA tolerance of xe2x89xa61 nm rms is required for low frequency errors, i.e. those of spatial wavelength of greater than 1 mm. Independent errors of each mirror must therefore be no greater than 0.25 nm, since in a system of N mirrors the maximum permissible error of each mirror is (2N)xe2x88x921 times the total error for the system. For mid-spatial frequency errors, of wavelength 1 mm to 1 xcexcm, surface roughness must be less than 0.2 nm rms as roughness in this spatial frequency range reduces image contrast. High-spatial frequency errors, of wavelength less than 1 xcexcm, cause large angle scattering, a loss mechanism for the beam, and so surface roughness for these frequencies must be less than 0.1 nm rms.
U.S. Pat. Nos. 5,986,795 and 5,420,436 both disclose the use of adaptive mirrors in photolithography using EUV radiation. In the mirror described in U.S. Pat. No. 5,986,795, a number of actuators are provided between a reaction plate and a face plate bearing a reflective coating suitable for the radiation used in the lithography apparatus. The actuators may be piezoelectric, electroresistive or magnetoresistive and act generally perpendicularly to the face and reaction plates. The reaction plate is more flexible than the face plate. U.S. Pat. No. 5,420,436 describes a similar arrangement, having an array of piezoelectric actuators acting perpendicularly between a reaction plate and a face plate; in this case however the face plate is more flexible than the reaction plate.
An embodiment of the present invention provides an adaptive reflector or system of reflectors, especially for extreme ultraviolet radiation, that can provide improved, control over the surface figure of the mirror and hence over wavefront aberration.
One aspect of the present invention includes a lithographic projection apparatus including a radiation system for providing a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate; and an active reflector comprised in an optical system being either one or both of said radiation system and said projection system, said active reflector comprising a body member, a reflective multilayer and at least one actuator controllable to adjust the surface figure of said reflective multilayer, wherein said actuator exerts a substantial force component in a direction parallel to the surface figure of said reflective multilayer.
The actuator in the active mirror serves to control the surface figure of the reflective multilayer and hence can be used to minimize wavefront aberration in the radiation beam delivered by the optical system. Stress, and particularly stress variations, have been identified as a major source of surface figure errors in reflectors adapted to reflect EUV radiation and the invention can directly compensate for this. The present invention can be used to compensate for stress inherent in the multilayer as a result of its manufacture as well as stresses caused by external factors. The actuators may be piezoelectric stack or patch actuators and are preferably incorporated into the reflector body close to the reflecting multilayer.
The actuators exert a substantial component of force in a direction parallel to the surface figure of the multilayer. In the case of a significantly curved mirror the force component should be parallel to the surface figure at or near the point of connection between the actuator and the multilayer or the member bearing the multilayer. The stiffness of the multilayer, or a member bearing the multilayer, is higher in directions parallel to the surface figure than in the direction perpendicular to the surface figure (note that in a local coordinate system having orthogonal x, y and z axes describing the mirror, the direction perpendicular to the surface figure at the center of the mirror may be referred to as the z-direction.). This means that a given force exerted parallel to the plane of the reflector effects a smaller deformation of the surface figure than the same force exerted perpendicularly. Since the required deformations are very small and actuators of the required strength are easily obtained, the present invention allows for a much more exact control of the surface figure, with reduced risk of over-deforming the mirror.
The actuators may lie wholly in the plane of the reflector, particularly where the actuators are patch actuators. However, the actuators may also be rod actuators arranged diagonally between the reflective layer and base member. In such an arrangement, the actuators may be arranged in pairs connected to the reflective layer at the same point but to the base plate at spaced-apart locations and controlled so that the resultant force exerted by each pair on the reflector layer lies wholly within the plane of the reflector. It is also possible for the actuators to be connected singly but in that case it is preferred that of the force exerted by each actuator on the surface figure, the component perpendicular to the surface is less than 50% and preferably less than 20% of the total force exerted by that actuator.
The actuators may also be arranged such that they are operative to apply torques to said active reflector so as to locally bend the reflective surface of the multilayer for controlling the surface figure. Applying torques proves very efficient in controlling the surface figure and advantageously the torques are about a point in or near the reflective multilayer. Torques may be applied by exerting the forces on projections of the active reflector at a backside opposing the reflective multilayer. In an embodiment the actuators applying the forces that induce the torques are arranged in between projections. The projections may be walls of cavities at the backside of the active reflector and a pneumatic or hydraulic pressure can be applied to the cavities so as to form pneumatic or hydraulic actuators applying forces to the projections. Generally, the projections will be substantially perpendicular to the surface figure and the forces exerted on the projections parallel to the surface figure.
According to a further aspect of the invention there is provided a device manufacturing method including projecting a patterned beam of radiation onto a target portion of a layer of radiation-sensitive material on a substrate, wherein at least one of a radiation system for producing the beam and a projection system for projecting the beam include an active reflector that includes a body member, a reflective multilayer and at least one actuator controllable to adjust the surface figure of said reflecting multiplayer, wherein said actuator exerts a substantial force component in a direction parallel to the surface figure of said reflective multilayer and controlling said active reflector to minimize wavefront aberration in a radiation beam reflected by said active reflector.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (XUV or EUV) radiation (e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.